Roofing granules are extensively used in roll roofing and asphalt shingle compositions. Such granules are generally embedded in the asphalt coating on the surface of an asphalt impregnated base material such as roofing felt with the granules thus forming a coating that provides an adherent weather resistant exterior roofing surface. As the outer granule coating also provides the aesthetic effect observable with respect to the roof to which the roofing material is applied, the appearance of the granules is of major marketing importance. For this reason, a pigmented color coat is ordinarily applied to the base mineral granules to enhance their visual decorative effect.
Kaolin clay is used extensively in silicate paint formulations for coloring roofing granules. It serves as a filler, extender, moisture release agent and reactant to aid film insolubilization during high temperature firing. Although clay is a major component of such coating formulations, it alone lacks the brightness and opacity needed to hide the dark underlying base rock of the granule. Although kaolin clay consists mainly of alumino silicates, other constituents are present as a result of the clays natural origin. Iron and titanium impurities, for example, impart a buff or yellow color to the clay while organic impurities such as humic acid derivatives cause a grey coloration observed in sedimentary kaolin. Unbound iron in the form of extraneous Fe (III) minerals also causes discoloration. For these reasons, white colored roofing granule coatings using natural kaolin clay frequently require appreciable amounts of expensive TiO.sub.2 to achieve desired color specifications.
White or light colored roofs are particularly favored in many areas. Titanium dioxide pigment is commonly used in the production of such white or light colored roofing granules. The TiO.sub.2 is commonly used in conventional insolubilized alkali silicate coatings, such as those described in U.S. Pat. Nos. 2,379,358 to Jewett, 3,255,031 to Lodge et al, 3,479,201 to Sloan, etc. As mentioned above, the kaolin clay which is frequently used in such coatings contains impurities such as iron, titanium and humic acid which tend to discolor the clay. Organic impurities, in particular, cause the clay to darken upon exposure to high temperature. This requires the use of larger amounts of TiO.sub.2 than would otherwise be necessary or desirable.
Various methods are known for increasing the brightness of kaolin clay. These include oxidation and/or reduction bleaching and high temperature calcining. Such methods are expensive. High temperature calcining, has a further disadvantage of producing clay which is unreactive toward silicate and therefore unsuitable for use in roofing granule coatings that are designed to cure by heat treatment alone. In addition, known techniques for brightening kaolin clay involve a separate treatment of the clay which would add a completely separate and unnecessary step to the normal roofing granule manufacturing process. Oxidation bleaching of kaolin using bleaching agents such as hypochlorite, ozone, chlorates, peroxides, perchlorates or perborates are among the known techniques for lightening kaolin clay by destroying discoloring organic impurities. All of these methods, however, substantially increase the cost of the clay.